1. Field of the Invention
The present invention generally relates to a flash memory and a method of forming a flash memory, and more specifically a method of forming a flash memory cell using an asymmetric control gate with a sidewall floating gate.
2. Description of the Related Art
A flash memory is unique in providing fast compact storage which is both nonvolatile and rewritable.
In a flash memory, the threshold voltage Vt for conduction of a field effect transistor (FET) changes state depending upon the amount of charge stored in a floating gate (FG) part of the FET. The floating gate is a charge storing region which is isolated from a more traditional gate conductor CG (control gate or "wordline") by a thin dielectric. The states of the Vt change with the amount of charge stored by the FG.
Since the FG directly controls conductivity between source and drain in a channel, the state of a FG memory cell is determined by applying certain voltages to the source or drain of the FET and observing whether the FET conducts any current.
Flash memory cells with a sidewall floating gate occupy a smaller area than those with conventional (layered) floating gates. For example, in U.S. Pat. No. 5,115,288, sidewall gates were formed on one side of the wordline by employing an extra mask. Sidewall spacers were formed on both edges of the wordline, then removed along one of the edges using the extra mask and an etching operation. Thus, the conventional approach uses a trim mask to define the floating gate.
However, this approach is expensive and requires good control of the overlay for the spacer removal mask.
Other conventional structures also are known. For example, in one structure, polysilicon spacers on both sidewalls are used for the floating gate. One spacer sits on top of the tunnel oxide area for programming, and the other one is called "added on floating gate". Both spacers are linked by a polysilicon body. However, a large cell size results.
In a second conventional structure, only one polysilicon spacer is used as the floating gate. A mask must be aligned to the top of the control gate, to remove the other floating gate spacer. Hence, the control gate cannot be small, since, otherwise, any misalignment will cause a problem. Therefore, this cell has difficulty in being down-scaled.
In yet another conventional structure, similar to the second conventional structure described above, a mask is needed to remove a sidewall spacer floating gate. Further, this spacer has a reentrance corner which is very difficult to be completely removed.
Thus, the conventional methods require extra process steps, material and more precise lithographic alignment, thereby resulting in increased manufacturing costs.